By Kim Deok-hyun SINGAPORE, June 5 (Yonhap) -- The United States is reviewing "additional options" to deal with North Korea after it torpedoed a South Korean warship, killing 46 sailors in March, U.S. Defense Secretary Robert Gates said Saturday.

"We are assessing additional options to hold North Korea accountable," Gates said in a speech at a security forum in Singapore, adding he won't elaborate what the new options would be.

During the speech, he described the North's deadly sinking of the Cheonan patrol ship as "provocative and reckless behavior."

"North Korea must cease its belligerent behavior and demonstrate clearly and decisively that it wants to pursue a different path," the defense secretary said.

Gates reaffirmed that the world must respond to the North's attack as Seoul officially took its first step to refer Pyongyang to the United Nations Security Council.

"Inaction would amount to an abdication of our collective responsibility to protect the peace and reinforce stability in Asia," Gates said.

On Friday, South Korean President Lee Myung-bak, who also attended the security forum in Singapore, formally appealed the Security Council to rebuke North Korea for attacking the Cheonan, Seoul's toughest measure to punish Pyongyang for the sinking.

Still, it remains unclear whether China and Russia, the North's traditional allies and two veto-wielding permanent members of the Security Council, would support South Korea.

In Singapore, President Lee and Defense Minister Kim Tae-young were stepping up efforts to win support from the international community for the South's responses in the aftermath of the sinking.

Kim and Gates met on Friday for the first time since the sinking. During the meeting, they agreed that "resolute" steps are needed over the North's sinking.

A multinational investigation concluded last month that the Cheonan was stuck by a North Korean torpedo before breaking in half and sinking in waters near their tense Yellow Sea border on March 26.